"FDB," Khloe wrote, which Rob promptly shared with his 5 million plus followers.
The slang term, which originates from rapper Young Dro's single of the same name, "F.D.B.," stands for "F— Dat B—-."
Unlike his big sisters—who chided Bailon via Twitter for kicking Rob "when he's down"—the Arthur George sock designer wrote nothing else regarding the subject matter.
Rob's subtle shade comes one day after Bailon, 30, slammed him in the September 2014 issue of Latina magazine for cheating while they were dating from 2007 to 2009. "He strategically planned things out so that he could cheat on me, and that to me was so disloyal," she told the mag.
The former lead singer of 3LW went on to say that appearing on the reality series while dating Kardashian was a poor move for her long-term career goals. "To be stuck with that Kardashian label, that was so hurtful to me and to my career," she said.
For access to all our exclusive celebrity videos and interviews – Subscribe on YouTube!